Process for purification of acrylic acid by fractional crystallization

Abstract

 A method of separating acrylic acid from an aqueous solution containing acrylic acid, comprising adding a salt that eliminates the eutectic point between acrylic acid and water, for example an alkali metal salt, in particular sodium chloride, to the aqueous solution in an amount sufficient to saturate the aqueous solution, and fractionally crystallising the aqueous solution to obtain acrylic acid.
The fractional crystallisation is preferably carried out in from three to six stages, and leads to a product which contains more than 99% of acrylic acid and less than 0.02% by weight of added salt, for example sodium chloride.

Description

[0001] This invention relates to a process for the purification of acrylic acid by fractional crystallisation.

[0002] Acrylic acid is currently manufactured by the vapour phase catalytic oxidation of propylene or acrolein. The gaseous reactor effluent from this process contains from about 10 to 80% by weight acrylic acid, with water, acetic acid and various organic impurities. This stream is then condensed or absorbed to obtain an aqueous solution of acrylic acid, acetic acid and water.

[0003] The purification of this stream to obtain acrylic acid has been extremely difficult. The relative volatilities of acrylic acid, acetic acid and water prevents the use of simple fractional distillation as a method of purification.

[0004] Several processes have been proposed for this purification. U.S. Patent No. 3,816,524 describes a method of separating acrylic acid by using various solvents to extract the acrylic acid from the aqueous solution. U.S. Patent No. 3,432,401 describes a method of using specific solvents as entrainers to aid in azeotropic distillation.

[0005] Both of these prior art processes suffer various disadvantages in that the temperatures necessary to perform distillation and solvent recovery can cause polymerization of the acid. Further, in extraction systems large amounts of solvent are necessary and such systems tend to have a high solvent loss.

[0006] As stated in U.S. Patent No. 3,432,401, fractional crystallization has not been considered as a means for recovering acrylic acid because a eutectic between acrylic acid and water exists at 63% by volume of acrylic acid. Below the eutectic point, the acrylic acid concentrates in a liquid phase, while above the eutectic, the acid concer- trates in a solid phase. The existence of the eutectic precludes complete separation of acrylic acid and water by direct fractional crystallisation.

[0007] Salts have been used in the process for the recovery of acrylic acid for various purposes. U.S. Patent No. 2,922,815 discloses that calcium chl:ride, sodium sulphate, and dry metal salts such as nickel chloride and nickel bromide have been used as drying agents to concentrate the acrylic acid present in the aqueous solution up to about 80%. U.S. Patent No. 3,846,488 discloses a method for accelerating the separation of an organic solvent phase fron an aqueous phase obtained by the extraction of acrvlic acid with a solvent by adding an alkali metal silt or ammonium salt in minute amounts to the aquecus solution.

[0008] Rather than accelerating separation betwem a solvent and water, or drying the aqueous solution, it has been discovered that the addition of cerain salts to the;aqueous solution removes the eutectic point between acrylic acid and water thereby aller- ing simple fractional crystallisation.

[0009] The invention therefore provides a process for separating acrylic acid from an aqueous solution containing acrylic acid, characterised in that a salt that eliminates the eutectic point between acrylic acid and water is added to the aqueous solution in an amount sufficient to saturate the aqueous solution, and the aqueous solution is fractionally crystallised to obtain acrylic acid.

[0010] Fractional crystallisation is a method of separating close boiling or azeotropic liquid mixtures. The method is a sequence of melting, partial freezing and separation stages. Often, fractional crystallisation can be more economic in energy consumption than distillation because latent heats of crystallisation are usually much smaller than latent heats of vapourization. A discussion of fractional crystallisation may be found in The Encylopedia of Chemical Technology 2nd. ed., Vol. 6, page 493.

[0011] Basically, fractional crystallisation can be used when two substances form a solid solution upon cooling. For example, a solution containing 50% by weight of component A when cooled, may precipitate a solid containing 65% by weight of component A. This solid can be separated from the liquid phase and reheated so that a liquid is formed having 65% by weight of component A. If this liquid is recooled, a solid may precipitate out that has 80% by weight of component A. As can be seen, by simply increasing the number of stages, up to 100% pure component A can be obtained. However, when two substances form a maximum or minimum melting point, a complete separation of the components is impossible. Acrylic acid and water are two such components. They form a eutectic at about 65% by volume of acrylic acid.

[0012] It has been discovered that the addition of certain salts has the ability to eliminate this eutectic and to allow fractional crystallisation.

[0013] The method for determining whether a specific salt has the ability to eliminate the eutectic is relatively easy. Basically, a concentrated aqueous solution of acrylic acid and water can be saturated with the salt and then cooled until a precipitate forms. Measurement of the relative acrylic acid concentrations in the remaining liquid and solid phases at a range of compositions should provide sufficient information to determine if the eutectic has been eliminated.

[0014] One such salt that will eliminate the eutectic is sodium chloride. Other alkali metal salts, the halides, nitrates and sulphates thereof, and ammonium salts also remove the eutectic. However, it has been found that barium chloride, ferric chloride, and tin chloride have no significant effect upon the acrylic acid-water solid-liquid equilibrium.

[0015] The amount of salt added to the aqueous solution must be sufficient to saturate the solution. For example, about 10% by weight of sodium chloride is required to saturate a 50% by weight acrylic acid solution.

[0016] After salt addition the aqueous solution should preferably be thoroughly mixed before being sent to fractional crystallisation.

[0017] The addition of the salt to the aqueous solution will depress the freezing points of the aqueous solution slightly. However, the temperatures required for fractional crystallisation are practical industrially. A solution saturated with sodium chloride can be fractionally crystallised over a temperature range from about -23°C to + 14°C.

[0018] Another advantage resulting from the addition of salt is that the separation per stage of fractional crystallisation is much greater than that of the acrylic acid/water system alone. This has the advantage of reducing the number of stages required and hence the cost for recovering acrylic acid.

[0019] An advantage in using a salt such as sodium chloride is that the remaining liquid from the crystalliser can be passed to an evaporator. Evaporation of the water makes possible the recovery and recycling of the salt.

Example - Crystallisation of Acrylic acid Solution

[0020] Crystallisation was carried out in a pseudo- fluid bed crystalliser. Mixing was accomplished by the bubbling of compressed air through a fritted glass disc that comprised the bottom of the crystalliser. The air was precooled by first passing it through a stainless steel coil immersed in the bath. After crystallisation, the remaining liquid was withdrawn by vacuum, with the dry crystals remaining on the glass disc.

[0021] Aqueous solutions with varying concentrations of acrylic acid were prepared. These solutions were then saturated with sodium chloride. The solutions were independently fed to the crystalliser, in which crystallisation took place. After separation, analysis of the liquid phase and the melted solid phase was performed by titration. The results are shown in the table below.

[0022] As can be seen in the table above, the eutectic point between acrylic acid and water has been eliminated, thereby allowing the recovery of pure acrylic acid.

[0023] It is anticipated that starting with an aqueous solution containing 50% by weight of acrylic acid, that 3 to 5 stages of crystallisation will result in 99% + acrylic acid. This acrylic acid product contains less than 0.02% by weight of sodium chloride. The saturated brine leaving the crystalliser contains about 14% by weight of sodium chloride which can be recovered by simple evaporation.

Claims

1. A method of separating acrylic acid from an aqueous solution containing acrylic acid characterised in that a salt that eliminates the eutectic point between acrylic acid and water is added to the aqueous solution in an amount sufficient to saturate the aqueous solution, and the aqueous solution is fractionally crystallised to obtain acrylic acid.

2. A process as claimed in claim 1 characterised in that the aqueous solution containing salt is mixed prior to fractional crystallisation.

3. A process as claimed in claim 1 or claim 2 characterised in that fractional crystallisation is carried out in one or more stages.

4. A process as claimed in claim 3 characterised in that fractional crystallisation is carried out in 3 to 6 stages.

5. A process as claimed in any of claims 1 to 4 characterised in that the aqueous solution is fractionally crystallised to obtain acrylic acid product stream and a brine stream containing the salt and water.

6. A process as claimed in claim 5 characterised in that the salt is recovered from the brine.

7. A process as claimed in any of claims 1 to 6 characterised in that the salt is an alkali metal salt.

8. A process as claimed in claim 7 characterised in that the alkali metal salt is a sodium salt.

9. A process as claimed in claim 8 characterised in that the salt is sodium chloride.